We have some "cardboard" stock at work that's as much as 1/4" thick and more akin to MDF than corrugated cardboard. Its pressed/laminated paper, and quite sturdy (stronger than MDF, and even some plywood). I have no idea if its available in lower densities or or other specifications. Its also difficult to cut, it likes to fuzz along cut edges if you try to saw or cut it with anything less than the sharpest blade available. A bandsaw works ok, but a jig saw just makes a mess. Trying to sand a cut edge it a waste of time, it just turns to fuzz. I suspect its cut with a large sheer as we receive it at work, although a CNC laser would work very well. I suspect it is cheaper than most plywood, and it is totally void free, so it might make for an interesting low-cost material. Of course the working properties and potential for extensive water damage probably makes it worse than crap plywood in most cases...

: I have epoxied some cardboard to see if it was possible to do a S&G kayak out
: of some quality boards (there must be different grades of cardboard) My
: problem was saturation of the inner segments of the board ... I just
: failed to develop confidence in the boards.

: But I still epoxy the outsides of cardboard boxes when I need a lasting,
: sturdy container. This may be better than watching epoxy cook off...